The present invention relates to color photography and particularly to color photography having special chromatic effects.
Color photography is based on the fact that the human eye is essentially receptive to three basic colors: blue (between 400 and 500 millimicrons), green (between 500 and 600 millimicrons) and red (between 600 and 700 millimicrons). The subjective reproduction of a given color, i.e., a certain distribution of light energy, can be obtained by the reproduction of the same average value of energy density in these three bands.
Any color photographic process necessarily involves the following two steps:
(a) analysis of the colors from the subject, and
(b) synthesis of the same colors
According to the mode of synthesis, one can distinguish between the additive and subtractive processes. In additive systems, the colors are formed by adding the complementary colors of the three fundamental colors, blue, green and red, found in the original subject. Upon looking at the image, the eye then receives the complementary color of that existing on the print, thereby restituting the original colors. The complementary colors of blue, green and red are yellow, magenta and cyan, respectively. Subtractive systems are practically the only ones still in use for photography.
Color selection can take place in two different manners:
(a) with three filters and a single emulsion sensitive to a broad spectrum in the visible, and
(b) with three emulsions sensitive to the three basic colors. In (a), filters can be incorporated into the emulsion in the form of a trichromic frame.
Color selection is usually obtained by using three emulsions, e.g., as in (b), above. Each emulsion is sensitive to a basic color. The emulsions are generally separated by thin transparent layers with or without filter action.
Color synthesis is obtained by selective generation of dyes (the usual case), by modifying dyes already present in the emulsion, by selective discoloration or by selective masking.
The formation of dyes involves a reaction between a developing agent and three complexing agents catalysed by the presence of the photo-reduced silver in the emulsion. This reaction includes three different steps:
(a) developer+n Ag.sup.+ .fwdarw.n Ag+oxidized developer
(b) oxidized developer+coupler.fwdarw.leuco dye
(c) leuco dye+oxidizing agent.fwdarw.dye Suitable developers are paraphenilenediamine derivatives having the general formula: ##STR1## wherein
R and R.sub.1 may be the same or different and are selected from lower alkyl groups such as --CH.sub.3, --C.sub.2 H.sub.5, and --C.sub.3 H.sub.7 ; and
R.sub.2, R.sub.3, R.sub.5 and R.sub.6 may be the same or different and are selected from alkyl groups and substituted and unsubstituted aryl groups.
Two types of couplers can be utilized: (a) phenol or naphtol derivatives leading to cyan dyes belonging to the indoaniline group; and (b) methylene derivatives having a methylene activated function and leading to yellow and magenta dyes of the azomethines group.
The reaction mechanism is as follows: ##STR2##
In basic pH, the developer molecule is oxidized by the Ag.sup.+ metal ion, thereby giving a diimine quinone ion stabilized by resonance: ##STR3## The next step is the reaction of the quinone diimine ion with the coupler ion. In basic pH the coupler is strongly ionized: ##STR4## which allows the coupling: ##STR5## The leuco derivative is then oxidized to form the dye: ##STR6## In the above formulae, A and B may be the same or different and can be selected from the following group: CH.sub.3, C.sub.2 H.sub.5, CH.sub.3 SO.sub.2 NHC.sub.2 H.sub.4, C.sub.2 H.sub.4 OH and C.sub.6 H.sub.5.
Emulsions with multiple layers provide color images by direct processing. The images can be negatives with complementary colors or positives after inversion. The structure is made of three superimposed emulsion layers separated by thin gelatin layers. These emulsions are, starting from the substrate:
(1) a red sensitized emulsion
(2) a green sensitized emulsion, and
(3) an unsensitized emulsion which is naturally sensitized to the blue.
The red sensitized emulsion is more sensitive than the green which is, itself, more sensitive than the blue, in order to compensate for the light absorbed in the preceding layers.
The unsensitized emulsion receives light first and absorbs the blue forming a latent blue image. The red and green are transmitted through the first layer. The green is absorbed by the second layer which forms a green image and transmits the red which is finally absorbed by the third layer. In order to avoid the transmission of the color corresponding to a sensitized layer to the next one, a dye is incorporated to that emulsion or to the separating layer. Such dyes are eliminated during processing.
The thickness of each sensitive layer may vary from 1 to 3 microns and the separating layers are 0.5 to 1 micron.
The peak sensitivites are:
UV to 450 nm for the blue, PA1 530 nm for the green, and PA1 630 nm for the red. PA1 yellow for blue PA1 magenta for green PA1 cyan for red PA1 magenta: 540 nm PA1 cyan: 660 nm
Unfortunately it is not possible to avoid a general sensitivity to UV.
Each elementary emulsion contains a dye-forming material (Coupler). The external emulsion sensitive to blue contains a yellow coupler; the middle emulsion sensitive to green includes a magenta coupler and the bottom emulsion, sensitive to red, contains a cyan coupler. The three dyes are formed during the processing and appear simultaneously together with the reduced silver. The images in each layer are developed in a color complementary to that for which it has been sensitized, that is:
The absorption peaks of the dyes are: p1 yellow: 440 nm
Special effects in color photography can generally be classified into 5 types: chromatic, geometrical, diffusion, structural and surface. Chromatic effects produce a color pattern different from the original (e.g. yellow skies, green faces, etc.). Such effects are generally produced by using filters, monochromatic lights during exposure or special processing techniques.
Geometrical effects provide a deformation of the shape of various components in the image (e.g. square wheels, oval faces, wrong perspective, etc.) very similar to the deforming mirrors found in amusement parks. Geometrical effects are obtained by special lenses, gratings, masks used in the exposure camera, duplicator or enlarger.
Diffusion effects provide diffuse edges, subrealistic images, unconventional shadows, etc. Such effects are produced by out-of-focus imaging, wide aperture or masking.
Structural effects can give a mosaic structure, structures similar to alligator skin, wood or other artistic structural effects. Structural effects are produced by double exposure or exposure through various transparent patterns.
Surface effects provide a painting-like finish or a metallic aspect and are produced by spraying the surface of the film or paper with an appropriate spray.
It is noted that all of the methods described above for providing special effects in the photograph require special equipment and/or processing.